The progress of laser in recent years has been remarkable and a high output and particularly, compact solid laser or semiconductor laser having a light emission region in the range from near infrared to infrared becomes easily available. These lasers are extremely useful as light sources for exposure in conducting the direct plate making from digital data of a computer or the like.
A photosensitive lithographic printing plate precursor for infrared laser to which infrared laser having a light emission region in the infrared region is used as the light source for exposure is a lithographic printing plate precursor containing, as essential components, a binder resin and an IR dye which absorbs light to generate heat. When the lithographic printing plate precursor for infrared laser having a positive-working photosensitive layer is exposed to infrared laser, in the unexposed area (image area) the IR dye in the lithographic printing plate precursor for infrared laser interacts with the binder resin to act as a dissolution inhibiting agent which substantially decreases the solubility of the binder resin. On the other hand, in the exposed area (non-image area), the IR dye absorbs light to generate heat and as a result, the interaction between the IR dye and the binder resin is weakened. Thus, during development, the exposed area (non-image area) is dissolved in an alkali developer to form a lithographic printing plate.
Such lithographic printing plate precursor for infrared lasers still have various problems. One of the problems is how to improve resistance to various chemicals for printing. When the chemical resistance of the surface is low, due to splash of the chemicals at the printing or residue of the chemicals used after wipe the image area changes in quality or dissolves to cause undesirably a problem in that printing ink becomes incapable of adhering during the printing. Another problem is how to improve scratch resistance of the surface. When the scratch resistance of the surface is low, the surface is rubbed with a member being touch with the surface during automatically performing exposure or development to cause scratch, whereby a problem of abrasion of the image area undesirably occur.
Various investigations have been made to overcome these problems. For instance, a technique of forming a recording layer containing an alkali-soluble resin having excellent chemical resistance is proposed (see, for example, JP-A-2005-62875 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) (corresponding to US2005/0037280 A1)). However, although the improvement in chemical resistance of the recording layer is recognized, circumference of a small scratch formed before development is dissolved at the development to cause a problem of occurrence of white spot in the image area.
Further, it is believed that two or more layer construction of the resin layer of lithographic printing plate is useful for achieving both the chemical resistance and the scratch resistance and, for example, use of a polymer compound containing as a constituting unit, a maleimide compound in a lower layer is proposed (see, for example, US2004/0067432 A1). However, adhesion of the lower layer to an upper layer is not necessarily sufficient and further improvements have been requested.